The invention relates to a locking device used for locking at least a piston associated with an operating element of a valve, a choke, a BOP (blowout preventer) or some other unit used in the field of oil/natural gas drilling and/or oil/natural gas production. The locking device comprises sleeve, which is axially displaceable relative to the hydraulic piston and which is threadedly engaged with a rotatably supported rod for displacement in an axial direction, said rod being drivingly connected to a motor such as an electric motor.
An example of a locking device is known from U.S. Pat. No. 7,300,033 B1. The locking device serves to lock a hydraulic piston which is used for displacing a slide of a BOP. The hydraulic piston is adapted to be axially displaced by means of a suitable hydraulic fluid in the open as well as in the closed position of the respective slide of the BOP. For locking the hydraulic piston at a specific position also without any hydraulic pressure being applied thereto, a lock rod is rotatably coupled to an electric motor. When the lock rod is rotated by the electric motor, a lock sleeve is displaced, said lock sleeve being threadedly engaged with the lock rod. The lock sleeve is brought into contact with a respective contact surface, normally in the interior of the hydraulic piston, so that the hydraulic piston will be fixed at said position by the displaced lock sleeve. Only when the lock rod is rotated once more for returning the lock sleeve to its starting position, thus releasing the hydraulic piston, said hydraulic piston can be displaced again by applying pressure thereto by means of the hydraulic fluid.
The locking device of U.S. Pat. No. 7,300,033 is disadvantageous insofar as it does not provide the possibility of detecting the position of the hydraulic piston and, consequently, the position of the slide of the BOP in real time and using it, if necessary, for locking by means of the lock sleeve. Even if the hydraulic piston should e.g. be located at a closed position, it is not exactly known how far the lock sleeve has to be displaced for fixing the hydraulic piston. This applies all the more to intermediate positions of the hydraulic piston between the open and the closed position of the respective slide.
Nor is it possible to find out whether a closed position of the hydraulic piston has actually been reached in response to the respective pressure applied thereto, or whether the hydraulic piston is prevented from arriving at its final closed position by other, in particular external influences. It is, however, normally necessary to precisely determine at least the closed position of the hydraulic piston so as to guarantee for said closed position that the slide of the BOP will actually be able to prevent a blow-out.
This applies analogously also to other positions of the piston and of the slide connected thereto.